There are many applications that require a rapidly curable coating composition that adheres to a substrate, is flexible, does not discolor and has low toxicity. For example, optical glass fibers are frequently coated with two superposed coatings. The coating which contacts the glass is a relatively soft, primary coating that must satisfactorily adhere to the fiber and be soft enough to resist microbending especially at low service temperatures. The outer, exposed coating is a much harder secondary coating that provides the desired resistance to handling forces yet must be flexible enough to enable the coated fiber to withstand repeated bending without cracking the coating.
Other applications, e.g., optical fabrication, coatings for substrates including glass, metal, wood, plastic, rubber, paper, concrete, and fabrics, and adhesives also require compositions that are fast curing, have low toxicity and provide good physical properties.
Compositions that include (meth)acrylate diluents have been utilized for many of these applications. However, (meth)acrylate diluents are hazardous to human health. Therefore, it is desirable to eliminate or reduce the amount of (meth)acrylate diluents present in a composition.
Vinyl ether compositions have been utilized as replacements for (meth)acrylates. Although vinyl ethers rapidly cure when exposed to ultraviolet light in the presence of a cationic curing catalyst, their cure under cationic conditions leaves catalyst residues that discolor the cured compositions and cause them to be sensitive to water. Furthermore, vinyl ether containing oligomers having relatively high equivalent weights, e.g., an equivalent weight in excess of about 500, do not cationically cure upon exposure to dosages of energy less than 3 Joules per square centimeter. Vinyl ethers do not homopolymerize in the presence of free radical initiators. Therefore, vinyl ethers are not suitable replacements for (meth)acrylates.
Unsaturated polyesters, e.g., maleates and fumarates, are known to be substantially non-toxic, but are unsatisfactory as replacements for (meth)acrylates because their rate of cure when exposed to ultraviolet light is not satisfactory for certain applications.
European Patent Application No. 0 322 808 published on 05.07.89 discloses a radiation curable composition that comprises an ethylenically unsaturated polyester component and a vinyl ether component having an average of at least two vinyl ether groups per molecule of the vinyl ether component. The unsaturated polyester component can be a polymer, oligomer or mixture thereof. Coatings produced from this composition are brittle and hard because of the large amount of ethylenically unsaturated groups in the backbone of the polyester component which leads to short chain segments between cross-links. The vinyl ether component reacts with the ethylenically unsaturated group and results in a high degree of cross-linking that causes the cured composition to be brittle, inflexible and hard. Thus, coatings produced from the composition of this European Patent Application do not possess the needed flexibility and softness for applications, such as optical glass fiber coatings, that require a flexible and soft coating.